Case Report Diffuse glioma – Rare homozygous IDH point mutation, is it an oncogenetic mechanism? Angad Singh, Mamta Gurav, Sandeep Dhanavade, Omshree Shetty and Sridhar Epari 1 Division of Molecular Pathology, Department of Pathology, Tata Memorial Centre, Parel, Mumbai, India Isocitrate dehydrogenase (IDH1/IDH2) mutations in glio- mas of WHO grade II/III and secondary glioblastoma are al- most always heterozygous missense mutations. Here, we report an extremely rare case of homozygous IDH1R132H mutation in a recurrent WHO grade III anaplastic astrocy- toma. The authors here also review the relevant literature for the possible metabolic impact of homozygous IDH1/2 mutations in the gliomagenesis. Key words: astrocytoma, DNA, homozygous point muta- tion, isocitrate dehydrogenase, sequence analysis. INTRODUCTION Isocitrate dehydrogenase (IDH1/IDH2) mutations were first identified during exome-wide sequencing of glioblas- toma as part of the cancer genome atlas project. They occur more frequently in grade II/III gliomas and secondary glio- blastomas which occur in a younger age group. IDH muta- tions occur in both astrocytic and oligodendroglial tumors; they are now known to be one of the earliest genetic alter- ations to occur in the development of diffuse gliomas. IDH mutations in gliomas are heterozygous missense mutations, most commonly in codon R132 of IDH1 or in the synony- mous codon (R172) of IDH2. Here, the authors report a rare case of somatic homozygous IDH1R132H mutation in an anaplastic astrocytoma and provide a review of the relevant literature. CLINICAL SUMMARY A 43-year-old man was first investigated in 2007 for an epi- sode of generalized tonic–clonic seizure (GTCS). He was then operated for a right frontal lesion, which was reported as low-grade glioma. However, no material of the same was presently available for histological review. He was kept on observation and was symptom-free for 9 years. Subse- quently, he again presented with an episode of GTCS and MRI showed a tumor recurrence in the right fronto-parietal region which was debulked. PATHOLOGICAL FINDINGS Histological features were of an infiltrating astrocytic tumor with areas of increased cellularity, nuclear anaplasia along with an occasional mitosis (Fig. 1A,1B). Necrosis and micro- vascular proliferation were not seen. On immunohistochem- istry, the tumor was positive for IDH1R132H (1:100, H07, Dianova GMBH, Hamburg, Germany; Fig. 1C), focally pos- itive for p53 protein (Fig. 1E) and showed loss of ATRX pro- tein (Fig. 1D) (1:750, polyconal, Sigma prestige antibody, Sigma, St Louis, MO, USA). MIB-1 labeling index was ap- proximately 5–7%. (Fig. 1F). MOLECULAR EVALUATION The tumor sample was also subjected to Sanger sequencing for IDH1R132 and IDH2R172 loci. DNA extraction, followed by PCR using specific primers for IDH1R132 and IDH2R172 loci was done. Gene-specific PCR and Sanger cy- cle sequencing subsequently performed revealed a homozy- gous mutation at codon 132 on exon 4 of the IDH1 gene (R132H). Adenine was seen in place of guanine at nucleo- tide 395 (c.395G > A), which was translating to histidine in place of arginine in protein sequencing (p.Arg132His) (Fig. 1G). Sequencing for IDH2R172 showed presence of ar- ginine (AGG) at codon 172 on exon 4 with no mutation. Gene sequencing performed on the patient’s blood also, showed wild-type IDH1R132 and IDHR172 sequences, thus excluding a germline mutation and confirming the homozy- gous genetic alteration seen in the tumor sample as a somatic homozygous mutation (Fig. 1H). Fluorescence in-situ hy- bridization (FISH) was also done to evaluate for chromo- some 2 (locus of IDH gene) number using a chromosome enumeration probe 2 (CEP2), which showed two signals suggesting diploidy and thus suggesting no copy number var- iation of IDH1 gene bearing chromosome 2 (Fig. 1-I); Correspondence: Sridhar Epari, Department of Pathology, Tata Memorial Hospital and ACTREC, Tata Memorial Centre, 8 floor, Annex building, Dr. E Borges road, Parel, Mumbai-400012, India. Email: sridhep@gmail.com Received 27 February 2017; revised 19 June 2017 and accepted 20 June 2017. © 2017 Japanese Society of Neuropathology doi:10.1111/neup.12401 Neuropathology 2017; ••, ••–••